Troia-Melides is a sandy area on SW Portugal, between Lisbon and Algarve, on the Atlantic Portuguese coast. The top north of the tract is a sandy peninsula about 10 kin long and 2 to 4 kin large, and is represented by recent dynamic sand-dune systems (Plio-Plistocene-Halocene) that extends south to the rocky Sines Cape, in a total-extension of 65 km (far beyond the zone covered by this work, that corresponds to northern half of this extension). The central zone of the littoral arch has sandstone cliffs headed by ancient dunes separated from the actual beach, which is continuous along the tract. A great diversity of important ecosystems characterises all the area, conferring it an enormous ecological fragility. Besides the dune and cliffs systems, 3 important coastal lagoons are also present. This diversity. of coastal environments was responsible for the definition of protected areas, classified according the more important species (birds, rare plants). Concerning flora, 45 families belonging to 246 different species where identified in a recent study. The major percentage occupies the inner dunes and includes several priority species (Directive 92/43/CEE) - Lonopsidium acaule, Thymus camphoratus, Linaria ficalhoana, etc. A scarce variety occurs in primary and embryonic dunes (being Ammophila arenaria the most common). This area has a great touristic potential, which is not always in conciliation with its fragile ecology. However, it's a relatively well-preserved tract and is nowadays under strict land management rules. In the study area, 3 touristic areas are approved, with a total number of 39,300 touristic beds, distributed by hotels and vacation houses. As a result, some delicate situations have occurred in natural ecosystems, namely aquifer exploitation, without regarding its recharge capacity, which lead to the infiltration of saline water in the aquifers of the littoral areas. With this work it was possible to understand the role of the saltwater intrusion due to the raise of buildings and fresh groundwater demands, and the previsible impacts on coastal ecosystems. Some procedures were undertaken in order to know the hydrogeochemistry and hydrodynamical characteristics of the aquifers in the area, namely defining the thickness of the fresh groundwater and the position of the interface with the saline water on the coastal area